1. Field of the Invention
The present invention relates to an optical information recording medium having an optical recording layer and a protective layer coating the optical recording layer.
2. Description of the Prior Art
An optical information recording medium comprises an annular substrate, such as a glass disc or a plastic disc, an optical recording layer, such as a metallic layer of Te, Bi or Mn, or a layer of a coloring matter, such as cyanine, merocyanine or phthalocyanine, and a protective layer coating the optical recording layer to protect the optical recording layer from being soiled and damaged.
In recording information in such an optical information recording medium, as shown in FIG. 5(a), a laser beam h.nu. is applied to an optical recording layer 2 from the side of a substrate 1 to form a pit 4 of about 1 .mu.m in diameter in the optical recording layer 2 by locally evaporating or sublimating the optical recording layer 2.
In reproducing the recorded information from the optical information recording medium, a laser beam having an energy smaller than that of the laser beam used for recording the information is applied through the substrate 1 to the optical recording layer 2 by a pickup head, not shown, to obtain a reproduced signal from the difference between the optical recording layer 2 and the pit 4 in reflectivity.
Another known optical information recording medium has an optical recording layer 2 formed of a coloring polymer or a metal carbide, in which the optical recording layer is deformed locally by applying a laser beam to the optical recording layer to form a pit. For example, the optical recording layer 2 has a smooth surface and a laser beam is applied to the optical recording layer 2 to deform the surface of the optical recording layer 2 locally in a convex shape or a concave shape as shown in FIG. 5 (b) to form a pit 4, or the optical recording layer 2 has a minutely rugged surface, and a laser beam is applied to the optical recording layer 2 to flatten the minutely rugged surface locally as shown in FIG. 5 (c) to form a pit 4.
The latter optical information recording medium has satisfactory recording characteristics and enables the reproduction of the recorded information with less noise as compared with the former optical information recording medium, because the latter optical information recording medium is free from noise attributable to the deformation of a hole forming the pit and chips scattered in forming the pit.
In reproducing the recorded information from such the former and the latter optical information recording media, part of the laser beam h.nu. penetrating the pit 4 is reflected by the protective layer 3 as typically indicated by arrows in FIGS. 5(a), 5(b)and 5(c), the reflected laser beam passes the pit 4 again, and then the reflected laser beam is detected. Accordingly, the difference between the quantity of light reflected by the pit 4 and that of light reflected by parts other than the pit 4 is small, and hence the C/N of the reproduced signal is small.
In the latter optical information recording medium having the optical recording layer 2 to be deformed for forming a pit, the surface of the optical recording layer 2 cannot be satisfactorily deformed when the surface of the optical recording layer 2 is coated closely with the protective layer, and thereby the recording sensitivity of the optical recording layer 2 is reduced.
To overcome such disadvantages inherent to the former and the latter optical information recording media, the conventional disc-shaped optical information recording media are formed in a construction of a so-called air sandwich type, in which two substrate 1 are joined together with a spacer 8 therebetween to form a gap 5 between optical recording layers 2 as shown in FIG. 6. However, this conventional optical information recording medium has a disadvantage in that the thickness thereof is considerably large. Consequently, it is impossible to form the optical information recording medium in a thickness not greater than 1.5 mm. Such an excessively thick optical information recording medium is unable to be used on conventional compact disc players and is unable to meet standards for the compact disc.